It is conventional that one may connect generators in parallel for supplying voltage to users. In this context, typically the power terminals of several series-wound generators designed for individual operation are connected in parallel without additional control devices or modifications. Load balancing has taken place, up to now, mainly by the internal resistance and by the negative temperature coefficient of the controller of the respective generator. In the case of conventional battery generator systems that are thermally tightly coupled, the temperature coefficient is used to ensure a sufficient battery load for all operating and temperature conditions. In these conventional systems, because of the internal resistance and negative temperature coefficient mentioned, a sufficiently balanced condition with regard to generator capacity utilization can be achieved. In this instance, the temperature coefficient has the following effect: The higher loaded generator heats up more than the less highly loaded generator. Because of the negative temperature coefficient, the output voltage of greater loaded generator drops, and with that, so does its load.
In the last few years, because of general advances in controller technology, the internal resistance of the controllers of the generators has been able to be significantly reduced, in part, by more than 80%. An additional trend in automotive technology led to a displacement of the battery from the engine compartment to places that are thermally uncoupled from the generator, for instance, to the trunk of the respective vehicle. Because of this, regulation of the charging voltage that was a function of the generator temperature became nonsensical. For this reason, increasingly, controllers are used that have a temperature coefficient of zero.
This lessening of the influence of the internal resistance and of the temperature coefficient as actuating variables leads to a balancing of generators that is clearly lower all the way to not being present any more, in response to their being connected in parallel.
Voltage differences between generators connected in parallel can come about, for instance, as a result of manufacturing variances, and lead to a different current output of the generators.
In the case of a different capacity utilization of the generators, increased wear of the more greatly loaded generator occurs as an undesired effect. This leads to an abbreviation of the service life of the overall system, which includes a plurality of generators that are connected in parallel.
A device for voltage supply in a motor vehicle, having generators connected in parallel, is described in German Published Patent Application No. 41 08 861. In this conventional device, a voltage regulator is assigned to each generator. In addition, the device has at least one battery whose one terminal can be connected to a voltage regulator via an ignition switch. Furthermore, a charge control device is provided that is in connection with the ignition switch and the voltage controllers, and is able to be connected via a switching device to the negative terminal of the battery which is preferably at ground. Between the charge control device, the voltage regulators and the switching device mentioned, there is a switching device having a plurality of components that conduct in one direction and block in one direction. By this device, it is supposed to be achieved that, by using two generators, high electric power can be generated, and the two generators and voltage regulators using a single indicator, for instance a charge control lamp, are able to indicate occurring errors in one of the two generator-voltage regulator systems, the other generator voltage regulator system being able to continue working without a problem.